The present invention relates generally to sensors, and more particularly to optical fiber sensor elements for heat and fire detection.
Fiber optic sensors are currently used to measure a wide range of parameters in distributed systems ranging from construction sites to aircraft wings. Some such sensors include pressure, strain, and temperature sensors, but fiber optics may qualitatively be used to measure any quantity that can be tied to the physical parameters of a fiber optic sensing element. Fiber optic temperature sensors, for instance, operate by detecting thermal expansion of a fiber optic strand, or of a surrounding sheath around, or gap between, strand segments with an interferometer. A data processor correlates this change in the physical parameters of the fiber optic sensing element with a corresponding change in temperature.
Most fiber optic temperature sensors comprise a fiber optic sensing element and an interrogator with a light source, a spectrometer, and a data processor. The sensing element consists of a fiber optic strand that extends from the interrogator into a sensing region. During operation, the light source of the interrogator emits light down the fiber optic sensing element. Changes in temperature alter the physical parameters of the sensing element, and thus its optical characteristics. The spectrometer and data processor assess these differences to identify changes in temperature.
Modern temperature sensors utilize a wide range of spectroscopy and interferometry techniques. These techniques generally fall into two categories: point and quasi distributed sensing based on Fiber Bragg Gratings (FBGs), and fully distributed sensors based on Raman, Brillouin, or Rayleigh scattering. The particular construction of fiber optic sensing elements varies depending on the type of spectroscopy used by the sensor system, but all fiber optic sensors operate by sensing changes in physical parameters of the fiber optic sensing element. Raman distributed sensors, for instance, determine a temperature from inelastic light scattering from thermally excited molecular vibrations within the silicon dioxide glass of the optical fiber core. Scattered light undergoes a spectral shift with respect to the wavelength of the incident light. This generates a higher wavelength Stokes component and a lower wavelength anti-Stokes component. The intensity of the so-called anti-Stokes band is temperature-dependent, whereas the Stokes band is almost independent of temperature. The temperature is derived from the ratio of the intensity of the anti-Stokes and Stokes components.
Many temperature sensing systems are required to detect both diffuse overheat conditions corresponding to region- or system-wide increases in temperature, and local overheat conditions corresponding to hot spots as small as 25 mm or less. To sense local temperature, conventional optical fiber sensing elements must have spatial measurement resolution at least as fine as the minimum heated length required to generate an alarm condition corresponding to such a hot spot. This high spatial resolution requirement increases the cost and complexity of optical fiber temperature sensors, and increases the time taken to acquire temperature measurements.